1. Field of the Invention
The present invention relates to an optical multiplexer/demultiplexer and a wavelength division multiplexing module.
2. Description of the Related Art
In recent years, an optical communication system using a silica fiber as an optical transmission line has been put into practical use, and it is required to reduce the size and cost for optical devices applied to this kind of optical communication system. Further, a wavelength division multiplexing (WDM) transmission system for transmitting a plurality of optical signals having different wavelengths through a single optical fiber has recently come to attention as a system capable of obtaining a greatly large transmission capacity and cost performance. In the wavelength division multiplexing transmission system, a band-pass filter having high-precision characteristics is required, so as to combine or separate a plurality of optical signals of close wavelength channels.
It is known that a band-pass filter, low-pass filter, high-pass filter, and other functions can be obtained by inserting in an optical medium propagating an optical beam an optical film composed of a single or plural layers having refractive indices different from the refractive index of the optical medium. For example, a band-pass filter used in an optical communication system is formed from an optical film obtained by alternately laminating low-refractive index layers and high-refractive index layers each having a predetermined thickness. The low-refractive index layers and the high-refractive index layers are formed of SiO.sub.2 and TiO.sub.2, respectively.
In fabricating the optical film, high-precision characteristics can be obtained by precisely controlling the thickness of the optical film as a whole or the thickness of each layer constituting the optical film. However, in a band-pass filter, for example, it is difficult to control a center wavelength in a pass band on the order of 1 nm by its fabrication technique only. To cope with this, not only the thickness of the optical film or each layer is controlled in its fabrication, but also an incident angle to the optical film fabricated is conventionally adjusted to change an equivalent thickness of the optical film or each layer, thereby finely adjusting the characteristics of the optical film.
In configuring an optical multiplexer/demultiplexer for optical signals having close wavelengths spaced about 1 nm from each other by using such an optical film, the characteristics of the band-pass filter formed from the optical film must be finely adjusted by adjusting the incident angle to the band-pass filter. However, a conventional mechanism for changing the incident angle to the optical film is apt to become large and complicated. Accordingly, an optical multiplexer/demultiplexer having such a mechanism becomes large in size and requires a high cost.